Field of the Invention
This invention is directed to a method and apparatus for testing hydraulically actuated safely systems such as blowout preventers for leaks.
Description of Related Arts Invention
Oil and Gas Exploration risk management includes the ability to control subsurface pressures which may be encounter during drilling operation. The primary mechanism utilized by operators to control downhole pressures is the hydrostatic pressure as a result of the drilling fluid contained within the wellbore. The drilling fluid is engineered and formulated to a density that provides a hydrostatic pressure inside of the wellbore that is greater than the formation pressure being drilled. In the majority of drilling operations, the hydrostatic control of wellbore pressure is adequate. However, from time-to-time the operator may encounter a higher than expected formation pressure where there is not adequate hydrostatic pressure to control the wellbore pressure. During these times the operator relies on a series of mechanical controls to stabilize the wellbore and prevent a “Blow Out”. A blow out is the uncontrolled release of fluid or gas from the wellbore. This event is extremely dangerous and therefore must be avoided if at all possible. The primary mechanical control device utilized by operators to control wellbore pressure is the Blowout Preventer (BOP) assembly. The BOP assembly consists of multiple sealing and shearing devices that are hydraulically actuated to provide various means of sealing around the drill string or shearing it off entirely, completely sealing the wellbore. It is essential that the BOP assembly operate as designed during these critical operations. Therefore it is a regulatory requirement to test the functionality and the integrity of the BOP assembly before starting drilling operations and at specific events during the drilling operations. The BOP assembly test is a series of pressure tests at a minimum of two pressure levels. low pressure and high pressure. During the pressure test, intensifying fluid from a high pressure intensifying pump unit is introduced into the BOP assembly in a volume sufficient to cause the internal pressure within the BOP assembly to rise to the first pressure test level. Once the first pressure test level is established the high pressure intensifying pump unit is isolated from the BOP assembly and the pressure is monitored for at least five minutes. Current regulations require that the pressure does not decay at a rate greater than 5 psi/minute or 25 psi total over the entirety of the five minute test. Upon successful completion of the first test a subsequent high pressure test is performed. The requirement for the high pressure test is the same as the lower pressure test. The pressure decay rate must not exceed 5 psi/minute or 25 psi total over the entirety of the five minute test. These tests are generally referred to within the industry as a hydrostatic test. Hydrostatic testing is a very well know and established practice and testing of BOP assemblies has been a required test for many years. The equipment utilized to perform the test has not changed over the years and is very dated. The typical fixed displacement hydrostatic test system utilizes a high pressure triplex plunger intensifying pump, driven by a diesel motor. The fixed displacement hydrostatic test system features a clutch assembly and a reduction gearbox between the diesel motor and the high pressure triplex intensifying pump. The drive ratio between the diesel motor and high pressure triplex intensifying pump is fixed and cannot be adjusted or changed once the hydrostatic test has been initiated. Some fixed displacement hydrostatic test system utilize an electric motor and variable frequency drive in place of the diesel motor and clutch, but otherwise operate similarly and have the same limitations related to their fixed displacement design. Additionally, the fixed displacement hydrostatic test system utilizes at least one pressure gauge and one chart recorder. The pressure gauge depicts the test pressure and the chart recorder records the pressure over time. The technician controls the pressure and pump rate by varying the diesel engine speed and by engaging or disengaging the clutch. Some units feature a multiple ratio reduction gearbox to increase the controllability of the fixed displacement hydrostatic test system when performing low flow rate test. The gearbox ration is manually selected by the technician and must be set before the test is performed. The entire hydrostatic test is manually controlled by the technician. A successful test relies entirely upon the skill of the technician and his ability to control the fixed displacement hydrostatic test system and interpret the pressure gauge. The reliance on the skill of technician and the lack of automation and computerization to enhance controllability makes the testing process problematic. In addition the mechanical chart recorder lacks the necessary resolution to make definitive pass or fail decisions. This requires the technician to utilize their skill and judgment when deciding if the BOP passed or failed the hydrostatic pressure test.
A more specific description of the currently utilized fixed displacement hydrostatic test system will reveal further short comings. A disadvantage of the currently utilized fixed displacement hydrostatic test system is the size of the high pressure triplex intensifying pump and the horsepower require to operate it. During a typical BOP assembly hydrostatic test the rate at which the intensifying fluid is pumped into the BOP assembly varies greatly with pressure. Initially the BOP assembly may contain substantial amounts of uncompressed air. Therefore the initial pump rate of a typical closed BOP assembly hydrostatic test might be 10 GPM but will decrease exponentially as the air is compressed. The “GPM” series of the chart in FIG. 1 depicts the exponential decrease of pump rate in relation to the pressure increase during a typical 10,000 psi BOP assembly hydrostatic test. Additionally, the “Horsepower” series of the same chart depicts the theoretical horsepower requirement related to the pump rate at the same pressure with the equation:Horsepower=Pump Rate (gpm)×Discharge Pressure (psi)/1714.
As clearly depicted in FIG. 1 the pump rate exponentially decreases as pressure increases and at approximately 1,000 PSI the pump rate is less than 1 GPM. Typical high pressure triplex intensifying pumps currently utilized in fixed displacement hydrostatic test systems applicable to BOP assembly testing have a maximum operating speed of 600 rpm. The displacement of the high pressure triplex intensifying pump is related to the maximum operating speed and the maximum designed pump rate. A typical 10 gpm high pressure triplex intensifying pump designed to operate at 600 rpm will have an approximate displacement of 3.85 cubic inches per revolution “cir”. The displacement of the high pressure triplex intensifying pump is fixed and therefore the torque to rotate the high pressure triplex intensifying pump at 10,000 psi is:Displacement (cir)×Pressure (psi)/75.4=3.85×10,000/75.4=510.61 ft-lbs torque.
Therefore, the theoretical horsepower to drive the high pressure triplex intensifying pump can be calculated with the equation:RPM×Torque (ft-lbs)/5252=600×510/5252=58.26 HP.
This differs greatly from the actual horsepower required and is a result of the fixed displacement design. Typical fixed displacement hydrostatic test systems do not provided a means of matching the displacement and the required pump rate. Therefore the torque requirement of the fixed displacement hydrostatic test system increase linear with pressure. The relationship between torque and pressure for the fixed displacement pump is depicted in FIG. 2. Another disadvantage of the fixed displacement hydrostatic test system is the lack of displacement resolution at higher pressures. For example, to pressure a BOP assembly, with an initial air volume of 10 gallons, from 1,000 psi to 10,000 psi only requires approximately 0.15 gallons of additional intensifying fluid to be added to the BOP assembly. This is less than 1 revolution of the high pressure triplex pump currently utilized on typical fixed displacement hydrostatic test systems. This is very difficult to control and the final pressure is often overshot. If the overshoot is large enough the test must be repeated. A typical state-of-the-art fixed displacement hydrostatic test system is approximately 10 ft long×5 ft wide and 5 ft tall. It is powered by a 75 hp diesel motor and weighs approximately 5,000 lbs. Also note the fixed displacement hydrostatic test system is manually operated with no provision for computerized operation or data collection. A typical hydrostatic test cycle utilizing a typical fixed displacement hydrostatic system commences with the technician pumping intensifying fluid at a high flow rate until the pressure gauge initially responds to the increasing pressure within the BOP system.
Once the initial volume of air is compressed (very low pressure) the pressure will increase very rapidly. Therefore, due to the lack of displacement resolution, the technician will begin to “bump” the fixed displacement hydrostatic test system to achieve the final pressure. “Bumping” is practice or technique where the technician cycles the fixed displacement hydrostatic test system on and off as quickly as possible using the clutch. This practice or technique relies heavily on the skill of the technician and can be very problematic and time consuming. It is also very easy to overshoot the test pressure. If the test pressure is exceed by a specified amount the test will not be valid and must be performed again. Lastly, the results are recorded on a manual chart recorder. The chart recorder is a very crude way of recording the test pressures and pressure decay rate (psi/min). A typical chart recorder has a resolution of 250 or 500 psi per line segment. While the chart recorder does provide a record of the BOP assembly hydrostatic test, it does not provided data about the actual leak rate.
As previously mentioned the current regulations, related to the integrity of the BOP assembly, requires the BOP to have a decay rate of less than 5 psi/min or 25 psi total over the entirety of the five minute test. It is reasoned by the regulators that if a BOP has a decay rate less than or equal to the maximum allowed by the regulation then it does not have a volumetric leak rate sufficient enough to compromise the functionality and integrity of the BOP assembly. Another reason for using a pressure decay model for BOP testing was the lack of any economically viable technology with a resolution capable measuring the volumetric loss related to leak rate. The loss of fluid associated with a leak of sufficient size to cause a 5 psi/min decay rate is miniscule. It could be less than 0.00002 GPM depending on the amount of air in the BOP assembly during the initial phase of the test. Measuring these extremely low flow rates accurately utilizing conventional flow meters is not practical or in some cases even possible. It is also evident that measuring the leak rate by monitoring the rate of pressure decay is inherently inaccurate. For example: if a typical BOP assembly with a volumetric loss rate (leak) of approximately 0.000008 gpm is first tested with approximately 5 gallons of air trapped within the BOP assembly during the initial phase of the hydrostatic test and subsequently tested with the same volumetric loss rate (leak) but with approximately 2.5 gallons of air trapped within the BOP assembly during the initial phase of the hydrostatic test, the BOP would pass the first test with approximately a 3 psi/min pressure decay rate but it would fail the second test with approximately a 6 psi/min pressure decay rate. Each test would have the same volumetric loss rate (leak) but the result of the tests would be significantly different. The effects of reduced initial air volumes in the BOP assembly increase substantially until at some point the pressure decay rate test will not be a viable means of leak detection. If the BOP assembly is hydraulically locked it will not be possible to utilize a pressure decay hydrostatic test. It is sometime the practice of the hydrostatic test technician to add air to the BOP assembly to ensure testability. This practice, while ensuring testability of the BOP assembly, will most likely lead to erroneous results as previously discussed above. Additionally, the resolution of the recorder makes it difficult to ascertain the actual pressure decay rate and the decision of pass/fail is mostly that of the technician's interpretation of the data. Subsequent to obtaining a successful test the chart recorder paper is signed and submitted as proof that the BOP assembly meets or exceeds the pressure decay rate specifications of the applicable regulations. Lastly, the entire intensified circuit is relieved of the intensified fluid via the dump valve. The typical dump valve is a manually operated needle or tapered plug valve.
Metal seat valves are used due to the extreme fluid velocities across the valve seat when the intensified fluid is released. Additionally, the intensified fluid flowing back from the BOP assembly carries contaminates such as sand and grit picked up from the BOP assembly. These valves must be serviced often to ensure the metal seat have not been comprised by the intensified fluid release. The much preferred current available soft seat designs lack the integrity to provide reliable service in this harsh service.
Consequently, there is a need for an improved hydrostatic test system that provides for fully variable displacement and is compact and easily portable. Such a system should also include a computer or processor to control and automate the test cycle and provide for useful data such as leak rate and other environmental and mechanical properties of the BOP Assembly. Additionally the test data should be electronically stored and easily disseminated via local and wide area networks in real-time or subsequent to the completion of the test.